1. Field of Invention
This invention relates to an electrostatic capacitance detection device capable of reading surface contours of a target object having minute ridges and valleys of a surface such as a fingerprint through detection of electrostatic capacitance which changes according to a distance from the surface of the object.
2. Description of Related Art
Related electrostatic capacitance detection devices used for fingerprint sensors and the like can be formed on a single crystal silicon wafer deposited with a sensor electrode and a dielectric layer set up on the sensor electrode Japanese Unexamined Patent Publication No. 11-118415, No. 2000-346608, No. 2001-56204, No. 2001-133213 and the like. FIG. 1 shows the principle of operation of a related electrostatic capacitance detection device. The sensor electrode and the dielectric layer form one of a pair of electrodes in a capacitor and the dielectric layer, and a human becomes the other electrode grounded. The electrostatic capacitance CF of this capacitor changes according to the ridges and valleys in a fingerprint contacting the surface of the dielectric layer. On the other hand, on the semiconductor substrate, there is prepared a capacitor forming the electrostatic capacitance CS, and these two capacitors are connected in series upon which a predetermined voltage is impressed. In this manner, there is generated a charge Q between the two capacitors, corresponding to the ridges and valleys in a fingerprint. This charge Q is detected using typical semiconductor technology and the surface contours of the target object are read.
However, these related electrostatic capacitance detection devices are formed on a single crystal silicon wafer, so that when too much finger pressure is applied at the time of using them as a fingerprint sensor, there was a breaking problem.
In addition, a size of approximately 20 mm×20 mm is required of a fingerprint sensor because of this application, hence, the sensor electrodes occupy a greater part of an area of the electrostatic capacitance detection device. The sensor electrodes are naturally formed on a single-crystal silicon wafer, while the major part of the single-crystal silicon wafer (the lower sensor electrode) created by expending enormous energy and labor serves a mere role of nothing but a supporting member. Namely, the related electrostatic capacitance detection devices are not only expensive, but also formed through huge waste and cost.
Further, it has strongly been suggested in recent years to attach a function of personal identification to cards such as credit cards and cash dispensing cards so as to enhance card security, whereas the related electrostatic capacitance detection devices made on a single-crystal silicon wafer lack flexibility, thus making it impossible to be formed on a plastic substrate.